The circuit of the present invention is useful in a number of different control functions including numerous types of liquid level control, maintaining light levels within predetermined limits, opening and closing garage doors, and any other functions where upper and lower limits can be detected by some type of means. In this application the invention is described in relation to maintaining the liquid level in a sump to assist in explaining how the invention operates. It should be understood, however, that the invention is not limited to sump pumps and may be incorporated into a wide variety of different devices in which some measurable quantity is to be kept between predetermined limits.
The prior art recognizes numerous types of control circuits for maintaining various mediums such as liquid level within predetermined limits. These circuits, however, usually require a steady consumption of electrical power even though they are simply in a standby condition which is wasteful of power and unacceptable in circumstances where batteries may be the power source. In addition, these devices often use complicated electro-mechanical switches which are troublesome and require much maintenance. The present invention overcomes these problems by the use of a unique new circuit as described hereinafter.